At present, there exists an increasing number of surgical interventions that benefit from the use of medical image based patient specific surgical guides as described for example in patent applications US 2005/0203528 A1 and EP 1 486 900 A1.
Thus, surgical guides are currently produced to give a unique anatomic position which corresponds to a patient specific pre-operative surgical plan. This plan is based on images of the patient with the assumption that the patient anatomy does not vary significantly within the time, maybe months after being imaged. If the patient's anatomy on which the guide was designed changes during this waiting period, the guide will not have a unique fit.
A second limitation of using a static, image-based guide is that the images may be unclear in certain regions. This has an effect in two ways: firstly, incorrect planning based on unclear anatomic landmarks is possible, for example a cut is deeper than expected; and secondly, the resulting guide does not fit in certain regions. With the static guide models complete surface matching is often not possible. Gaps or hinges can be introduced between the guide and the patient anatomy due to imperfect image segmentation or a change in patient anatomy between the imaging date and the surgical date.
Surface hinges are most detrimental to the guide fit since they limit the remaining surface contact and the unique fit is then impossible. To compensate for possible hinges large gaps are intentionally made over these regions so that the majority of the surface can still make contact and produce a proper fit. However, with less surface contact available, the likelihood of an improper fit even increases. This improper fit can lead to incorrect surgical results since the active elements are in a different position than what was pre-operatively planned.
Often, windows are added to a guide so that the surgeon can check for gaps along the guide surface to determine the quality of the fit. This creates a compromise between fit visibility and use of a possible locking surface area. The more locking surface used to make the guide, the less visibility the surgeon has as to see if the fit is good or not.
A third limitation of the current surgical guides is that they transfer a surgical plan that is only based on the images and intraoperative visual information. However, in some interventions there are good reasons to adjust the original plan during surgery based on tangible information. The tension inequality in the ligaments of a joint (e.g. ligament balancing in knee surgery) is impossible to visualise in most situations, but it is an element considered by many surgeons.
The result is that multiple surgical acts are difficult to perform in a guided way and hence, in fact, finally rely on mental navigation once a difference from the pre-planned patient anatomy is determined. In some medical conditions, adjustable guides have been conceived to allow for some variability intra-operatively to adjust the referenced functional elements like guides for drill holes or cut planes. (U.S. Ser. No. 12/039,849) These modifications to the pre-operative planning have to be done manually by shifting or rotating active elements of the guide so that they align visually to certain anatomic structures or landmarks. It would be possible to define a range of probable surgical results based on the maximum rotation or translation of the active components. However, the exact point within the range is determined by visual references and is made manually. These adjustable guides also have the same risks as static guides in that they can be misplaced on the anatomic surface since they use a static surface contact. If this occurs, the surgeon can no longer trust any pre-operative planning since the reference position is inaccurate.
Thus, the adjustment can be needed but is problematic for the same reasons as those that affect the fit of guides, but also due to the fact that the position of the body of the patient may be quite different on the operation table compared to the time when the image scans were taken.
An alternative approach to the problem is to perform some surgical acts that need high accuracy whilst the patient is being scanned. Theoretically this gives the best guiding feedback to the surgeon, because he can see in real time what he is actually doing. However imaging for long periods during an operation, adds its own risks for the patient and the medical staff, e.g. radiation exposure.